**2. Materials and methods**

#### **2.1 Location of the study nursery**

The experiment was carried out in summer in a 2-year-old apple tree nursery (*Malus communis va*r. Golden Delicious, Rosaceae) located near the National School of Agriculture in Meknes (GPS coordinates: 33° 50′ 37″ N, 5° 28′ 39″ W) in plastic tunnel under ambient conditions. It is installed inside an orchard of the same culture.

### **2.2 Origin and physicochemical composition of olive mill wastewaters**

The vegetable water samples were collected from a traditional olive oil extraction unit located in the Sefrou-Morocco region (GPS coordinates: 33° 49′ 50″ N, 4° 50′ 15″ W). Sampling was carried out immediately after olive oil extraction and OMW production; the effluents were taken, on three times with an interval of 15 min between two successive sampling. The samples were brought back to the Laboratory of the National School of Agriculture in Meknes in airtight boxes and stored at 4°C to avoid any alteration of their chemical composition.

OMW analysis (pH, electrical conductivity or EC, crude protein level, potassium, phosphorus, magnesium, calcium, copper, manganese, iron, zinc, sodium, cadmium and nickel) was carried out at the Official Laboratory of Chemical Analysis and Research, Casablanca Morocco (www.loarc.ma) according to standard methods. The content of suspended solids, the chemical oxygen demand (COD), the biological oxygen demand (BOD), concentrations of nitrites, ammonium, sulphate, nitrate, orthophosphat and chlorine were determined at the Laboratory of the Department of Basic Sciences of the National School of Agriculture in Meknes as methods described in Rodier et al. [52]. The total polyphenol content was determined according to the method of Folin Ciocalteu [53, 54]; to this end, 200 μL of OMWW were mixed with 1 ml of a freshly prepared Folin–Ciocalteu reagent (10 times diluted) and 0.8 ml of 7.5% sodium carbonate (Na2CO3). The whole mixture

was incubated for 30 minutes at room temperature (20° C). The reading was carried out using a UV–visible spectrophotometer (Shimadzu 1601, Europe) at a wavelength of 765 nm. The concentration of polyphenols is expressed in milligrams of gallic acid equivalent per gram of OMW).
